The goals of this research are (a) to describe the early development of empathic, prosocial behaviors and aggression in children and (b) to begin to identify both biological/constitutional and environmental factors that contribute to individual differences. A behavior genetics, longitudinal research design is used to compare young monozygotic and dizygotic twins. The procedures for this study are based on naturalistic and experimental methods constructed earlier for a longitudinal study of the emergence of prosocial behaviors and aggression in children during the second year of life. Clear developmental progressions in children's empathic, prosocial responses to others in distress were identified in the initial study. For example, 1 1/2- to 2-year-old children (but not 1 year olds) were able to assist persons in distress, whether the child had caused (i.e., aggressed) or witnessed the problem. These behavior patterns, in turn, were linked to experimentally assessed transitions in social-cognitive processes reflecting self-recognition and perspective-taking. The research procedures, then, were applied to a sample of 150 monozygotic and 150 dizygotic pairs of twins, 14 months old when the study began. Developmental patterns in twins are similar to those of singletons. Behavior genetics analyses of heritability of empathic, presocial behavior and aggression are being conducted. Preliminary analyses of prosocial behaviors in a sub-sample of the twins suggest evidence for heritability in empathic responsiveness to others' emotions: Monozygotic twins are more similar to one another than dizygotic twins (for females only). Young girls appear more emotionally responsive than young boys. Subsequent analyses will focus also on heritability of aggression.